Wet-type multi plate clutches have been commonly used in the power trains for automotive vehicles and for the power take off shafts (PTO shafts) of many forms of earth moving equipment and agricultural equipment. A wet-type multi-plate clutch is suitable for such pieces of equipment which are frequently and abruptly engaged and disengaged under heavy load, but is known to involve some dragging due to the viscosity of the lubricating oil which adheres to the clutch plates. Shifting of a power transmission device is sometimes made difficult by this dragging and the attachment which is connected to the PTO shaft may not stop immediately after the clutch for the PTO shaft is disengaged. Furthermore, dragging of the clutch when it is disengaged involves unnecessary power loss by friction and is therefore undesirable in this respect also.
In order to eliminate such an inconvenience, Japanese Patent Publication No. 49-30047 proposes a brake device which comprises a friction plate which is interposed between a clutch housing and a driven shaft, and a hydraulic actuator provided in the clutch housing for applying braking force to the friction plate.
However, the provision of such a braking device for preventing dragging of a clutch means addition of a large number of component parts to the clutch, increased complexity of the structure and higher manufacturing cost. Furthermore, the braking device must be properly lubricated for preventing excessive wear and heat generation of the brake device in addition to lubricating the clutch plates of the clutch.
A multi plate clutch is capable of transmitting large power for its size but has been known to lack the capability of producing a half engaged state which is favorable for starting off a vehicle, as compared with a single plate clutch. Therefore, an inexperienced operator may have some difficulty in starting off a tractor equipped with such a clutch.